Please be aware that this is a quite heatedly discussed subject. One side says "yes, this is how to do it", the other "no, that is just wrong". So please decide for yourself what "side" to choose (and of course decide case by case).
Many people map areas using new nodes. This might be the (seemingly) most easy way but it has several disadvantages when the area in fact is "attached" to nearby streets in the real world:
a) When moving the streetnodes around, the area has to be adjusted by hand. That's unnecessary extra work.
b) It's gluttony, the right nodes already exist.
c) Due to not connecting street and area there might un-intended gaps when rendered.
How to map:
Simply start your area and click on the already existing nodes, it's like magic!
How to edit:
If you have an area like above there is no "free" side of it to click on, if you want to select it. No idea about Potlatch here. In JOSM press and hold middleclick on any side, then press and hold CTRL, leftclick on the desired way in the popup.
How to fix:
Enter The Great Merge Tool (M in JOSM). It seems to be rather dick-headed in its behaviour though... If it merges to the wrong node just undo, zoom in a lot and try to place both points just above each other. Maybe there is a way to tell it how to merge, but shouting at it is quite satisfying.
And now shout at me!